[jira] [Created] (LUCENE-7617) Improve GroupingSearch API and extensibility

["Alan Woodward (JIRA)" <ji...@apache.org>]

Alan Woodward created LUCENE-7617:
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             Summary: Improve GroupingSearch API and extensibility
                 Key: LUCENE-7617
                 URL: https://issues.apache.org/jira/browse/LUCENE-7617
             Project: Lucene - Core
          Issue Type: Improvement
            Reporter: Alan Woodward
            Assignee: Alan Woodward
            Priority: Minor


While looking at how to make grouping work with the new XValuesSource API in core, I thought I'd try and clean up GroupingSearch a bit.  We have three different ways of grouping at the moment: by doc block, using a single-pass collector; by field; and by ValueSource.  The latter two both use essentially the same two-pass mechanism, with different Collector implementations.

I can see a number of possible improvements here:
* abstract the two-pass collector creation into a factory API, which should allow us to add the XValuesSource implementations as well
* clean up the generics on the two-pass collectors - maybe look into removing them entirely?  I'm not sure they add anything really, and we don't have them on the equivalent plan search APIs
* think about moving the document block method into the join module instead, alongside all the other block-indexing code
* rename the various Collector base classes so that they don't have 'Abstract' in them anymore



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